An electret material including an electrode plate and an electret layer formed thereon has conventionally been used in electrostatic acoustic transducers such as earphones, headphones, and microphones. For example, in an electret capacitor microphone (ECM) having a built-in IC (integrated circuit) element, an electret material is disposed so as to face the front-side or back-side surface of a diaphragm.
Various processes for producing such an electret material have been proposed. For example, Patent document 1 describes a process in which a thermoplastic-resin film capable of constituting an electret layer is laminated to a metal sheet and the film is converted to an electret. Patent document 2 and Patent document 3 describe a process in which a dispersion including a dispersion medium and fine particles of an FEP (specifically, a tetrafluoroethylene/hexafluoropropylene copolymer) dispersed therein is applied to a back electrode plate and heated to thereby form a thin film and this thin film is converted to an electret.